关于 rsync 的新发现

我使用rsync从ext4硬盘将文件复制到FAT32硬盘（使用FAT32是为了兼容Windows和Mac）

复制的过程中，我发现rsync没有增量传输，而是每次都重新传输

查了一下，得知FAT磁盘的时间存储位比ext少，导致 rsync 认为两边文件时间不一致

据微软所说，NTFStoFAT也有这个问题

When files are copied from NTFS drives to FAT drives, some file time stamp rounding has to occur; the file time stamp is rounded up to the next even second.

...

NTFS time stamp: 7 hours 31 min 0 sec 001.

FAT time stamp becomes 7 hours 31 min 2 sec 000.

至此我才明白，rsync只检查时间和文件大小，就决定两边文件是否一致

如果构造两个文件，内容不一样，但是时间和大小都一样，rsync就会认为两文件一样从而不再传输

比如，创建两个文件，一个内容为 123，一个内容为 321

$ cat file1.txt file2.txt
123
321

然后把他们 touch 成时间相同

$ touch file1.txt file2.txt

然后看一下他们的详细信息

$ stat file1.txt file2.txt
  File: file1.txt
  Size: 4               Blocks: 8          IO Block: 4096   regular file
Device: 10302h/66306d   Inode: 7471107     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/z)   Gid: ( 1000/z)
Access: 2021-05-14 11:27:13.168183020 +0800
Modify: 2021-05-14 11:27:13.168183020 +0800
Change: 2021-05-14 11:27:13.168183020 +0800
 Birth: -
  File: file2.txt
  Size: 4               Blocks: 8          IO Block: 4096   regular file
Device: 10302h/66306d   Inode: 7471108     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/z)   Gid: ( 1000/z)
Access: 2021-05-14 11:27:13.168183020 +0800
Modify: 2021-05-14 11:27:13.168183020 +0800
Change: 2021-05-14 11:27:13.168183020 +0800
 Birth: -

然后我们执行rsync

$ rsync -vv -n --no-whole-file --inplace file1.txt file2.txt
delta-transmission enabled
file2.txt is uptodate
total: matches=0  hash_hits=0  false_alarms=0 data=0

sent 46 bytes  received 76 bytes  244.00 bytes/sec
total size is 4  speedup is 0.03 (DRY RUN)

rsync果然没有传输

针对这种情况，还需要加上--ignore-times参数，才能让rsync传输时间、大小都一致的文件